Will the zinc coating create galvanic corrosion to non-coated black steel when in contact? A reviewer of our project insisting to provide a transition from galvanized pipe to black steel pipe. I can't find any listed dielectric fitting for 6" pipe and I'm not sure if there is a such a thing.

Where necessary to join metal pipe with pipe of dissimilar metal, the joint shall be insulated against passage an electrical current using an approved method.

Rubber gasket joints (push on or mechanical joints) are considered not connected electrically.

The joint in threaded fitting would be bare steel for both the coated pipe and black pipe. Are you really contacting two dissimilar metals? It is always best practice to have no dissimilar metals in contact or to insulate the metals from one another.

I have seen dielectric flange fittings use to connect the underground fire sprinkler piping from the aboveground piping where the electrical code requires bonding of the above ground piping to the the electrical grounding system. This prevents the under ground piping from becoming a grounding electrode and the current between the soil and pipe corroding the piping.